1. Field of the Invention
The present invention relates to a droplet ejecting apparatus and a method for driving the apparatus.
2. Description of the Related Art
In recent years, inkjet printers are desired to include a capability of stably squirting tiny droplets at a higher frequency to print a high-resolution image at a higher speed.
A phenomenon of break-off of an ejected droplet is one of causes that degrade image quality. A droplet ejected from a nozzle of a liquid ejecting head is followed by a tail (hereinafter, “satellite”) extending from a meniscus of the nozzle. This satellite can break off from the droplet into flight. The satellite portion broken off from the meniscus flies as a satellite droplet (while the droplet that flies first is referred to as “principal droplet”).
As the viscosity of ejected liquid increases, this satellite produced at droplet ejection increases in length. In particular, when a droplet that is small in volume (generally approximately 3 picoliters or smaller) is ejected, because a difference in dot diameter between a satellite droplet and the principal droplet is small, the satellite becomes undesirably relatively conspicuous. Presence of such a satellite droplet degrades image quality. Furthermore, satellite droplets exert a large influence on image quality particularly when a configuration that includes a plurality of heads is employed. This is because if satellite droplets are produced differently among the heads, the satellite droplets change color tone by making difference in brightness or the like.
Furthermore, other problems can also arise. For example, reading accuracy of a bar-code can deteriorate when printed with satellites. A text image can degrade in image quality (more specifically, be blurred) when printed with satellites. In a case where satellites are considerably small in volume or fly at a low velocity, the satellites are gradually diffused as mist, in which case probability of occurrence of a problem, such as internal contamination with ink of a printing apparatus where a heed(s) is mounted, increases.
Against this background, a technique related to a single-pulse drive waveform configuration P3 for suppressing satellite production at ejection of a tiny droplet is conventionally known. The waveform configuration P3 includes a first contracting waveform component r1 that causes a principal droplet to be ejected, a fixed-duration-holding waveform component d2 subsequent to the waveform component r1, and a second contracting waveform component r2 to be applied after the waveform component d2 invariably at timing application at which amplifies oscillation of a meniscus generated by the waveform component r1. This configuration amplifies a satellite without exerting an influence to velocity of a principal droplet, thereby reducing a length of the satellite.
Japanese Patent No. 4770226 discloses a technique including detecting an environmental temperature of a head, and applying to a piezoelectric element a drive waveform that is stretched or compressed in a direction of a voltage axis and a direction of a time axis depending on the detected environmental temperature. A second pulse, which is a reverberation adjusting component subsequent to an ejecting component, is optimized by changing a width or timing of the second pulse in such a manner that: the lower the environmental temperature, the more the reverberant oscillation is amplified; the higher the environmental temperature, the more the reverberant oscillation is damped.
However, the waveform configuration P3 described above is disadvantageous in the following respect. To further reduce the length of the satellite, a voltage Vr2 of the second contracting waveform component r2 can be raised, or there can be employed a waveform configuration P2+P3 by adding a plurality of ejection pulses P2 (generally at resonance intervals of Tp=1Tc) antecedent to the waveform component r2. The waveform configuration P2+P3 allows ejecting a droplet of a large liquid amount (hereinafter, “large droplet”) by merging droplets ejected by the ejection pulse P2 and the satellite-shortening ejection pulse P3. The waveform configuration P2+P3 amplifies oscillation of the meniscus relative to oscillation produced by application of the pulse P3 singly. Because oscillation produced by application of the second contracting waveform component r2 is further superimposed on the oscillation, frequency characteristics degrade. In addition, unexpected unnecessary droplet can be ejected by the second contracting waveform component r2. Even when such an unintended droplet is not ejected, there arises a problem that the second contracting waveform component r2 can cause the meniscus to unnecessarily bulge and induce distortion or the like of a droplet ejected in a next period, thereby notably degrading image quality when driven at a high frequency.
Furthermore, in a high-temperature condition where residual oscillation is less prone to damp, the second contracting waveform component r2 amplifies the oscillation by a degree larger than required, thereby notably degrading image quality when driven at a high frequency as in the above. There is also another problem that, in a low-temperature condition where residual oscillation is prone to damp, effect of the satellite shortening is not obtained because residual oscillation necessary to push out a satellite portion is not produced.
To solve the problems described above, a crest value of the second contracting waveform component r2 can be lowered in a high-temperature condition. However, this causes the meniscus to be pushed less by compression of a liquid chamber and results in failure to obtain a second satellite shortening effect, which will be described later, provided by neck formation in an ink column. Furthermore, because a second expanding waveform component f2 for lowering the voltage back to an intermediate voltage is also reduced, it becomes difficult to enhance a residual-oscillation damping effect. When, on the other hand, the crest value of the waveform component r2 is increased in a low-temperature condition, the number of troubles such as ejection of an unnecessary droplet increases sharply, which leads to a problem of notable degradation in image quality as in the case described above.
Furthermore, another waveform component for damping meniscus oscillation that is amplified in a high-temperature condition or when a large droplet is ejected may be added to a trailing end of P3 to prevent degradation in frequency characteristics. However, such addition not only complicates waveform but also increases a length of the waveform, and therefore prevents increasing a printing speed.
The technique disclosed in Japanese Patent No. 4770226 is disadvantageous in that, when residual oscillation of a meniscus velocity is damped, bulge of the meniscus is also undesirably reduced and, accordingly, the satellite shortening effect to be provided by neck formation in an ink column is also lessened. Therefore, attaining both of satellite shortening and stable ejection is difficult.
Under the circumstances, there is a need for a droplet ejecting apparatus that minimizes influences on a drive waveform length and a waveform configuration, is highly stable, has favorable frequency characteristics, and is capable of ejecting droplets with fewer satellites even in a condition where an environmental temperature varies relatively greatly.